1. Field of the Invention
The present invention relates to a suspension in a vehicle such as an automobile, and more particularly to a fluid pressure type active suspension.
2. Description of the Prior Art
As shown in, for example, Japanese Patent Laying-open Publication 62-187609 it is known to incorporate a fluid pressure type active suspension in a vehicle such as an automobile which comprises a fluid pressure type actuator provided between a vehicle body and a wheel, a fluid supply passage for supplying a working fluid to said actuator, a fluid exhaust passage for exhausting the working fluid from said actuator, a pressure control means traversing said fluid supply passage and said fluid exhaust passage so as to control supply and exhaust of the working fluid to and from said actuator so as thereby to control the fluid pressure in said actuator, and a control means for controlling said pressure control means in accordance with running conditions of the vehicle.
Further, as described in, for example, the specification of Japanese Patent Laying-open Publication 2-133217 assigned to the same assignee as the present application it has already been proposed to construct the pressure control means of the above-mentioned type so as to comprise a release passage for releasing a very small part of the working fluid from said fluid supply passage and fixed and variable throttle means provided in said release passage so as to thereby generate a pilot pressure controlled by the variable throttling effect of said variable throttle means and to operate a switching-over valve in the pressure control means by said pilot pressure so as thereby to control the pressure in said actuator by the control of the variable throttle means.
Such a pressure control means is diagrammatically illustrated FIG. 15 of the attached drawing.
Referring first to this FIG., 300 and 302 designate a pump and a reserve tank, respectively, and 304 and 306 designate a pressure control means and an actuator, respectively. The pump 300 is connected by a fluid supply passage 308 with a port 312 of a spool valve 310 serving as a switching-over control valve in the pressure control means, while a port 316 of the spool valve is connected with the reserve tank 302 by a fluid exhaust passage 314. A passage 320 connects a working fluid chamber 318 of the actuator 306 with a port 322 of the spool valve 310. An accumulator 324 is connected to the fluid supply passage 308, and a gas spring 326 is connected to the passage 320.
The spool valve 310 has a housing 328 and a valve element 330 disposed to be reciprocable in said housing. Pilot chambers 332 and 334 and an annular space 336 are defined by the housing and the valve element.
The fluid supply passage 308 is connected with the fluid exhaust passage 314 by a release passage 338, and a fixed throttle means 340 and a pilot valve 342 are provided to in the release passage in said order. The pilot valve comprises a solenoid 344, a valve element 346 and a variable throttle port 348 the opening of which is varied in accordance with positioning of said valve element by said solenoid. A valve chamber 350 formed on the upstream side of the variable throttle port of the pilot valve is connected with the pilot chamber 332 of the spool valve 310 by a pilot passage 352. The pilot chamber 334 of the spool valve 310 is connected with the passage 320 by a pilot passage 354.
Therefore, when the opening of the variable throttle port 348 is increased by a control of the electric current supplied to the solenoid of the pilot valve 342, the pressure in the valve chamber 350 is lowered as a result of increase of the pressure drop across the fixed throttle means 340. Along with lowering of the pilot pressure Pp in the pilot chamber 332 the valve element 330 moves downward in the figure, and accordingly the port 316 is connected with the port 322, and therefore the pressure in the working pressure chamber 318 of the actuator 306 lowers. On the contrary, when the opening of the pilot valve 342 is decreased, the pilot pressure in the pilot chamber 332 rises, so that valve element 330 moves upward in the figure, so that the port 312 is connected with the port 332, and therefore the pressure in the working fluid chamber 318 rises.
In the fluid pressure type active suspension incorporating such a pilot type pressure control means the consumption of the working fluid increases when the vehicle runs on a rough road or makes a continuous slalom driving, and if as a result the amount of the working fluid in the accumulator 324 decreases, the pressure in the supply passage 308 lowers. Therefore, when the control operation of the pressure control means is continued under such running conditions of the vehicle, the pressure in the fluid supply passage 308 will approach a target pressure to be set up in the working fluid chamber 318 or would become even lower than such a target pressure under certain circumstances.
Under such operating conditions the pilot valve 342 gets to a fully closed or a almost fully closed state, and therefore the flow of the working fluid through the pilot valve decreases to a very low flow value, and therefore the relation between the force generated by the solenoid 344 to bias the valve element 346 upward in the figure and the pilot pressure in the chamber 350 to bias the valve element 346 downward in the figure becomes nonlinear, resulting in an unstable operation of the pilot valve so that the valve element of the pilot valve vibrates. If it happens, the pilot pressure fluctuates, thereby causing vibrations of the valve element 330 of the spool valve 304, thereby causing fluctuations of the pressure in the working fluid chamber of the actuator, thereby causing vibrations of the actuator, resulting in vibrations of vehicle body and/or noise generation.